In a continuous ink jet printer, a continuous jet of electrically conductive ink is expelled from a microscopic orifice in a print head to form an ink filament. The ink jet is stimulated by a periodic disturbance induced by a stimulation signal applied to the ink jet head to cause the ink jet to reliably break up into an evenly spaced series of drops. An electrode located in the vicinity of the drop break-off point is employed to induce a controlled amount of charge on the ink jet filament. The induced charge is trapped on the ink drop as it separates from the filament, and the trajectory of the ink drop is determined by the interaction between the charged drop and local electric fields. In a binary type ink jet printer, drops are either charged or not. Charged drops are deflected along a catch trajectory into an ink drop catcher and uncharged drops proceed undeflected to an ink receiving surface such as paper. In another type of continuous ink jet printer, drops are selectively deflected along a plurality of printing trajectories, or a catch trajectory, depending upon the level of charge imparted to the drops by the charging electrodes.
In such continuous ink jet printers, the velocity of the ink drops can vary greatly due to variations in ink pressure in the ink jet print head, and variations in the viscosity of the ink. The ink viscosity may vary due to changes in temperature, due to evaporation of the solvent in the ink, or due to variations from batch to batch in the ink formulation.
It is desirable to control the velocity of ink drops in the ink jet printer because velocity of the drops has an influence on the size of the drops, and the momentum of the drops (which affects splashing). It is known to attempt to control the velocity of ink drops by keeping the ink pressure constant with an ink pressure servo. Unfortunately, changes in ink viscosity due to temperature and evaporation make this approach unsuccessful. Another approach, as outlined in U.S. Pat. No. 3,787,882 issued Jan. 22, 1974 to Filmore et al is to attempt to measure directly the velocity of the ink drops and to control the ink pressure to maintain a desired velocity. The apparatus disclosed by Filmore et al includes a pair of inductive sensors arranged along the line of flight of the charged ink drops. Circuitry is provided for measuring the time of flight of a group of charged drops between the two inductive sensors, to thereby determine the velocity of the drops. The ink pressure is adjusted by a servo loop with velocity as a feedback signal to produce a predetermined velocity.
One problem with such an arrangement is the sensitivity of the measurement that can be achieved. Because the sensitivity is low, a group of charged drops is employed to increase the signal. However, this reduces the accuracy of the measurement due to the uncertainty in the exact location of the drops. Furthermore, because the inductive sensors are in the vicinity of the drop charging electrodes, a large component of nose is induced on the sensors by induction from the drop charging electrodes. Another shortcoming is the need for interposing extra structures (the inductive sensors) in the region of the drop charging electrodes, an area where space is extremely limited in modern high resolution ink jet printers.
It is the object of the present invention to provide a means for sensing the velocity of ink drops in a continuous ink jet printer and for adjusting the ink pressure in response to the measured velocity that is free of the shortcomings noted above.